Win condition: Defeat commander
Your sole foe is an unknown Cantor, but don't count this battle out – it's not exceedingly challenging, but it can be time consuming. Your foe will summon terrors repeatedly, which means you need to make it your main priority to deal damage in spades, enough to counter his heals each turn.
If you can, send a powerful until within close proximity of the Cantor, but don't attack, just wait – the Cantor will not attack you directly unless provoked, and will instead focus on summoning its terrors. Wear away the Cantor's health with your mages and Thunder (his own magic can't match Thunder's range), and once his HP is low enough, finish him with your unit in melee range. Be careful, as your melee hit rate will likely be quite low, possibly below 50%, and his retaliation will do a number on you. If you can, keep Genny nearby to heal. You can also use illusory soldiers here, and while they're a great help at keeping the terrors at by and providing Mage and Cleric protection, they also make it more difficult to access the Cantor than it already is. Use your best judgment.
Once the Cantor's health is finally low enough, finish him to complete the map. You'll be rewarded with Fruit of Life and a massive XP bonus – if you're bold you can try to strategize who will finish him, but the safe route is to simply ensure victory with your reliable units.